1. Field of the Invention
This invention relates generally to network switching devices and more particularly to Fibre Channel switching devices and the dynamic credit allocation for a port based on the port-to-port link distance.
2. Description of the Related Art
The Fibre Channel family of standards (developed by the American National Standards Institute (ANSI)) defines a high speed communications interface for the transfer of large amounts of data between a variety of hardware systems such as personal computers, workstations, mainframes, supercomputers, storage devices and servers that have Fibre Channel interfaces. Use of Fibre Channel is proliferating in client/server applications which demand high bandwidth and low latency I/O such as mass storage, medical and scientific imaging, multimedia communication, transaction processing, distributed computing and distributed database processing applications. U.S. Pat. No. 6,160,813 to Banks et al. discloses one of the state of the art Fibre Channel switch systems, which is hereby incorporated by reference.
One or more interconnected switches form a network, called a fabric, which other devices, such as mass storage devices, servers or workstations, can be connected to. Any devices connecting to a fabric can communicate with any other devices connected to the fabric. A direct connection between two devices is a link. An interface on a device for connecting another device is a port. A non-switch device connecting to a fabric is a node on the network or fabric. A port on a non-switch and non-hub device is an N-port. A port on a switch may be an E-port, for connection to another switch port, an F-port, for connection to an N-port, an FL port for connection to an FC-AL loop or any combination of the above. A link between two switches is an inter-switch link (ISL).
Each port has a transmitter and a receiver. The transmitter sends out information and the receiver receives incoming information. There are buffer memories associated with each port, either the transmitter or the receiver, to temporarily store the information in transit, before the information is confirmed to be transmitted towards its destination by a switch, or to be stored or used by a device at its destination. The buffer memory can be in the actual port or, preferably, may be centralized, as shown in U.S. Pat. No. 6,160,813. The buffer memory is broken down into units. One unit of buffer memory, which can hold one frame, is represented by one buffer-to-buffer credit or one credit. A frame is a unit of information transmitted, which comprises a header portion and a payload portion. The header portion identifies the frame, including a Source Identification (SID) and a Destination Identification (DID). The payload portion contains the data being transmitted. A frame may be 2112 data bytes long, which, plus header, CRC, EOF totals 2148 bytes.
In the prior art, a receiver on a port is allocated a fixed amount of buffer space to temporarily store received frames, represented by a fixed number of buffer-to-buffer credits. The receiver controls the allocation of the buffer space. At the initial configuration when two switches connect, the receivers advertise to the transmitters the amount of buffer space represented by the number of credits available. The transmitters initialize their credit counters to the number of credits advertised by the receivers. Both the transmitting port and receiving port keep track of the use of the buffer space using the number of credits and credit counters. Each time a frame is received by the receiver, the frame is stored in a buffer space and the number of credits residing in the receiver is increased by one. The transmitting port keeps track of this by reducing its transmitter credit counter, which indicates how many more frames can be sent, while the receiver increments its receiver credit counter, which indicates how many frames are stored in the buffer space. Once the frame is confirmed to have been retransmitted by a transmitter on the receiving switch, or used by a device, then the buffer space is free to be used to store a new frame. At that time, a credit is returned by a transmitter on the receiving port to a receiver on the transmitting port and the receiver credit counter in the receiving port is decreased by one. When the transmitting port receives the credit, the transmitter credit counter in the transmitting port is increased by one.
Even though frames travel through the fiber optics at the speed of light, it still takes time for frames to move from one device to another. It also takes time for a device to receive a frame; process it or retransmit it; and then return a credit, i.e. a confirmation of receipt, back to the transmitting port. It takes some more time for the credit traveling through the optical fiber to reach the transmitting port. During the turn-around time between when the transmitting port sends out a frame and the transmitting port receives a credit, the transmitting port may have sent out a number of frames at its transmitting speed if the transmitting port has available credits. When the transmitting port has at least a minimum number of credits to allow the transmitting port to continue transmitting until it receives credits back from the receiving port, the effective frame transmission rate is the highest. If the transmitting port does not have that minimum number of credits, then it has to temporarily stop sending frames when all the credits are used and wait until the credits return. Due to this stoppage, the effective frame transmission rate may be substantially lower than the actual transmission rate. That minimum number of credits depends on the turn-around time and the frame transmitting speed. The longer the transmission line, or the faster the transmitting speed, the more frames that may be in transit. At a fixed transmitter speed, the more credits a port can have, the longer the transmission distance can be while the link still maintains the full effective transmitter speed.
In the prior art, the number of credits allocated to a port is fixed, but the distance between ports may be different. For long distant links, there may not be enough credits to sustain the full speed of the transmitters. For shorter distance links, there may be more credits than necessary such that some of the buffer space or credits are wasted. Even for Fibre Channel switches where the buffer memory are centrally allocated and controlled, the amount of credits or buffer space allocated to each port is still fixed. To alleviate such problem, in some other prior art Fibre Channel switches, an installer of the physical port to port link may manually configure the buffer space dedicated for a particular port depending on the distance between the port to port link. There may be discreet distant levels that an installer can select, such as 5, 10, 50, 100 kilometers. Still many times these levels are too far higher than the actual link distance deployed by the Fibre Channel network installer. For a given configuration, a particular distance level setting may be wasteful because it will over-commit the buffer credits based on next higher level. With the advancing in the speed of Fibre Channel switches and the distance of the Fibre Channel links, the demand for buffer space or credits is increasing. The inflexibility of buffer space or credits allocation becomes increasingly costly.
Therefore, it is desirable to have a method to match the credit demand for a port connecting to a port to port link to the available credits on a Fibre Channel switch. It is also desirable to have devices to implement such a method.